880 BULLETIN OF THE BUREAU OF FISHERIES, 



2. MEASUREMENT OF THE WATER FLOW. 



If the flow is small and so delivered that the volume flowing during a few 

 seconds or half a minute may be caught in containers, it may be measured directly. 

 If the flow is too large for this or is through ground pipes, conduits, or ditches 

 in one plane, other means must be resorted to. Technical methods, by the use 

 of current meters, the pitometer, or weir measurements may be used where 

 available. It is intended to discuss here only the simple methods open to anyone 

 without the use of technical instruments. 



Where the water is delivered into a pond of reasonably regular shape, it is 

 often easy partly to draw off the water, measure the space thus drawn off, and 

 calculate its cubic contents. The pond may then be allowed to fill up to the 

 original mark and the time required noted. A fair estimate of the flow per 

 minute may thus be readily obtained. If the delivery is below the surface of 

 the water in the full pond a slight error is introduced by the change of head, 

 which is decreasing as the water rises. This error may be minimized by lowering 

 the pond only a few inches, or the least distance that will permit an estimate to 

 be made. Often this estimate may be checked by the following method: 



. When the flow passes through a completely filled closed conduit the cubic 

 contents of which may be measured, it is sometimes feasible to determine the 

 speed of the flow through this conduit. This may be done by observing the 

 time required for an object to be carried entirely through the conduit by the 

 current, the instant of its entering and leaving this current at each end of 

 the pipe being accurately noted. A block or ball of wood floating tlirough 

 the upper portion of the conduit is not a good instrument for determining the 

 speed of the flow, being apt to scrape the inner surface of the conduit and be 

 retarded; besides, the current is slowest next the surface and fastest in the 

 center. A round, short-neck bottle may be weighted with shot and tightly 

 corked, so that its specific gravity is almost exactly the same as that of water, 

 and when completely submerged it will neither rise nor sink. It will thus 

 remain (in shallow water) at about whatever depth it is placed, for a consider- 

 able time at least. If the bottle is of such length that it approaches the diam- 

 eter of the conduit, say three-fourths of the diameter, it will, after starting 

 properly, float submerged three-fourths of the cross section of the pipe, be thus 

 acted upon by the currents of different rate and give a fair basis for the average 

 speed of the water in the conduit. From this speed and the cubic contents 

 of the conduit the volume delivered per minute is easily calculated. 



Flow in open flumes or ditches can easily be measured. Simple modifica- 

 tions of the above method, which it is unnecessary to detail, will readily suggest 

 themselves. 



